Optimize array_positions() for scalar needle #20769

Is your feature request related to a problem or challenge?

Analogous to the way in which array_position was optimized (#20530), we can modify array_positions() to do a single large arrow_ord::cmp::not_distinct over the entire flat values buffer and then reconstruct the results in a single pass for the entire batch, rather than computing results on a row-by-row basis.

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